Field of the Invention
This invention relates to data transmission systems used between computers or like terminals, for inter-board connection, or for data interfaces in data transmission systems, switching systems, etc.
In many data links in which digital data packets are transmitted between computers or boards, a specific synchronization pattern called a "delimiter" is added to the leading end or to both the leading and trailing ends of each packet to realize packet synchronization. Particularly, where the packets are variable in length, reliable detection of the delimiter is necessary for the recognition of the leading and trailing packet ends.
As an application of an optical fiber communication to the data link of a such system, there are fiber channels as disclosed in, for instance, Fiber Channel Physical Layer (FC-PH) Rev. 2.1, ANSI X3T9.3/91-071, May 25, 1991). For parallel digital data transmission, the parallel data is often converted through time division multiplexing into a serial signal for transmission of the data on a reduced number of data lines. In such data transmission systems, it is necessary for the receiving side to realize frame synchronization for the serial-to-parallel conversion in order to ensure that the parallel data channels inputted on the transmitting side and those outputted on the receiving side are in accord with one another. To realize frame synchronization, a specific synchronization pattern is applied, which enables the receiving side to detect the frame phase.
In such a data link, a first problem is encountered when a transmission error occurs in the synchronization pattern itself which is added to data to be transmitted. In this case, it is unable to recognize the packet or frame phase, thus making all the data contained in the packet or frame ineffective. The problem arises frequently, because in an optical data link such as fiber channels, the transmission line is basically not error-free. In the case where the length of the packet or frame is fixed, it is possible to avoid the problem considerably by adopting the so-called "forward protection", i.e., causing a synchronization pattern detector operation like that of a flywheel (for instance, Kaneko, "PCM Communication Techniques", pp. 109-116, Sanpoh, 1976). Where the length of the packet or frame is variable, however, such a method cannot be adopted. On the contrary, there are cases where the same pattern as the synchronization pattern is present and can be erroneously judged to be the synchronization pattern. To avoid this, in the usual data link data is encoded such that the same pattern as the synchronization pattern will not be present in the resultant data. For example, in the case of the fiber channels mentioned above, transmission line codes called 8B10B codes are used, and only predetermined data characters are used for the data section. Since the delimiter has a pattern which is not contained in any data character, usually there is no possibility of mistaking data as the delimiter. Even in such a case, however, the occurrence of a transmission error in the data gives rise to the possibility of the appearance of the same pattern as the delimiter.
A second problem that arises in parallel data link is that of transmission time differences or skews among the individual channels. In a well-known method of absorbing skews, a FIFO (first-in first-out) buffer is provided on the receiving side of each channel, and data is read out from each FIFO under control of an equal phase clock. This method poses a problem that the hardware is increased.
Further, in the case of transmitting parallel data after parallel-to-serial conversion, there is a requirement for frame synchronization on the receiving side. For the restoration to the original parallel data it is necessary to discriminate each channel after the serial-to-parallel conversion and make a feedback control to the frame phase at the serial-to-parallel converter or effect channel interchange. In the former method, the control requires a certain period of time. During this time, no data can be transmitted. For instantaneously establishing the frame synchronization, therefore, the latter method has to be used. As methods of channel discrimination, there is one in which a unique synchronization pattern is used for each channel (a similar method being disclosed in Japanese Patent application Kokai Publication No. Hei 04-115734), and one in which channel identifiers are added to data. In the former method, a common structure cannot be adopted for a pattern detection circuit for each channel, thus leading to high cost. The latter method, on the other hand, has a problem of transmission efficiency reduction.